J/A+A/565/A21 Astrometric catalog around ultracool dwarfs (Lazorenko+, 2014)
Astrometric planet search around southern ultracool dwarfs.
II. Astrometric reduction methods and a deep astrometric catalogue.
Lazorenko P.F., Sahlmann J., Segransan D., Martin E.L., Queloz D., Mayor M.,
Udry S.
<Astron. Astrophys. 565, A21 (2014)>
=2014A&A...565A..21L 2014A&A...565A..21L
ADC_Keywords: Positional data ; Stars, double and multiple ; Proper motions ;
Parallaxes, trigonometric
Keywords: astrometry - technique: high angular resolution -
atmospheric effects - parallaxes - brown dwarfs
Abstract:
We describe the astrometric reduction of images obtained with the
FORS2/VLT camera in the framework of an astrometric planet search
around 20 M/L-transition dwarfs. We present the correction of
systematic errors, the achieved astrometric performance, and a new
astrometric catalogue containing the faint reference stars in 20
fields located close to the galactic plane. Remote reference stars are
used both to determine the astrometric trajectories of the nearby
planet search targets and to identify and correct systematic errors.
We detected three types of systematic errors in the FORS2 astrometry:
the relative motion of the camera's two CCD chips, errors that are
correlated in space, and an error contribution of yet unexplained
origin. The relative CCD motion has probably a thermal origin and
usually is 0.001-0.010px (0.1-1mas), but sometimes amounts to
0.02-0.05px (3-6mas). This instability and space- correlated errors
are detected and mitigated using reference stars. The third component
of unknown origin has an amplitude of 0.03-0.14mas and is independent
of the observing conditions. We find that a consecutive sequence of 32
images of a well-exposed star over 40min at 0.6arcsec seeing results
in a median r.m.s. of the epoch residuals of 0.126mas. Overall, the
epoch residuals are distributed according to a normal law with a
χ2 value near unity. We compiled a catalogue of 12000 stars with
I-band magnitudes of 16-22 located in 20 fields, each covering 2x2'.
It contains I-band magnitudes, ICRF positions with 40-70mas precision,
and relative proper motions and absolute trigonometric parallaxes with
a precision of 0.1mas/yr and 0.1mas at the bright end, respectively.
This work shows that an astrometric accuracy of ∼100 micro-arcseconds
over two years can be achieved with a large optical telescope in a
survey covering several targets and varying observing conditions.
Description:
Astrometric catalogue of 12000 faint stars in 20 sky fields with
ultracool dwarfs listed in Table 1 of the paper. The data were
obtained by optical imaging with the FORS2 camera at the Very Large
Telescope with I-Bessel filter. The catalogue contains magnitudes,
ICRF positions, relative proper motions, and absolute parallaxes for
16-22mag stars within ∼1 arcminute from the dwarf.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 20 20 Target list (ultracool subdwarfs)
cat_fors.dat 118 12943 Deep astrometric catalogue
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See also:
J/A+A/556/A133 : Astrometric orbit of DENIS-P J082303.1-49120 (Sahlmann+, 2013)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Nr [1/20] Sequential number
4- 12 A9 --- ID Short identifier, which refer to approximate
field sky position
14- 17 F4.1 mag Imag I-band magnitude (Bessel filter, center=768nm)
19- 20 I2 --- Ne [9/16] Number of observing epochs
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Byte-by-byte Description of file: cat_fors.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Nr [1/20] Field number (1)
3- 7 I5 --- Seq [1/1915] Sequential star number in the field
9- 14 F6.3 mag Imag [13.8/25.1] I-band magnitude (2)
16- 20 F5.3 mag e_Imag Standard uncertainty of I-band magnitude
22- 32 F11.7 deg RAdeg Right Ascension J2000.0, epoch J2000.0 (3)
34- 44 F11.7 deg DEdeg Declination J2000.0, epoch J2000.0 (3)
48- 52 F5.3 arcsec e_RAdeg Standard uncertainty of RAdeg
56- 60 F5.3 arcsec e_DEdeg Standard uncertainty of DEdeg
62- 69 F8.2 mas/yr pmRA Proper motion in RA, pmRA*cosDE,
relative to field stars
71- 78 F8.2 mas/yr pmDE Proper motion in DE, relative to field stars
80- 85 F6.2 mas plx [-19/106] Parallax, absolute (4)
88- 91 F4.2 mas/yr e_pmRA [0/5.4] Standard uncertainty of pmRA
93- 96 F4.2 mas/yr e_pmDE [0/5.2] Standard uncertainty of pmDE
98-101 F4.2 mas e_plx [0.04/4.4] Standard uncertainty of PLX
103-111 F9.4 yr Ep [2011.4/2012.2] Mean epoch of observations
113-118 F6.2 --- Qual [0.1/306] χ2 of the epoch residuals (5)
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Note (1): Sequential number of the field with ultracool dwarfs listed in Table1.
Note (2): zero-point of magnitudes was found using DENIS photometry (DENIS
Consortium, 2005, Cat. B/denis).
Note (3): conversion from local coordinate system to ICRS was made using
USNO-B catalogue (Monet et al., 2003, Cat. I/284)
Note (4): conversion from the relative to absolute parallaxes are made using
zero-points by Sahlmann et al. (2014, 2014A&A...565A..20S 2014A&A...565A..20S)
Note (5): this is the reduced χ2 of the epoch residuals which flags the
quality of the data. Large χ2 (over 3) indicate low precision due to
binarity of the object, a close background star, CCD defects, etc.
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Acknowledgements:
Petro Lazorenko, laz(at)mao.kiev.ua
References:
Sahlmann et al., Paper I, 2014A&A...565A..20S 2014A&A...565A..20S
(End) Patricia Vannier [CDS] 03-Apr-2014